I actually do not think it should be so hard to determine where a "line" should be.
In my belief, there is a very distinct (and recognizable) difference between taking advantage of a poorly designed offer, and finding a way to actively rob due to a failure in the systems that the provider has.
If an entity ABC offers X points per credit card purchase, then finding a way to make 1c purchases is, in my book, a stretch of the friendship, but not fraudulent.
But as per a recent thread, when people discover that if they buy something and get X points, then know that if they return the item that they get the points despite not paying ANYTHING, then this is a scam.
Maybe it is too subtle a difference for some, but in this latter case the actions are so deliberately false that it aggrieves me that this forum is used as the medium to perfect these thefts.
I love cheap fares. I love when I can discover city-pairs that gain me cheap good seats. I get when you can buy cash with your credit card and earn points. But I do not like scams.